1. Field of the Invention
The present invention relates to a compressor for obtaining high pressure compressed gas, and particularly relates to a multistage compressor.
2. Prior Art
Conventionally, compressed gas has been used as a power source for operating various machines, and there has recently arisen a pronounced desire for more and more highly pressurized gas. In order to meet with this desire, multistage compressors have conventionally been employed in many cases.
One example of such multistage compressors is shown in FIG. 1.
The multistage compressor shown in the figure is for supplying compressed air. The compressor comprises a compressor body including: a lower pressure compression part 3 or first cylinder receiving a reciprocating piston 1 therein, and a higher pressure compression part 4 or second cylinder receiving a reciprocating piston 2 therein, and an intermediate conduit 5 connecting lower and higher pressure compression parts 3 and 4, respectively, with each other. The compressor further comprises a manually operable lower pressure side unloader or first unloader apparatus (not shown) associated with the lower pressure compression part 3 for keeping the inlet valve 6 of the lower pressure compression part 3 opened when the first unloader is actuated, a tank 7 connected with the higher pressure compression part, and a higher pressure side unloader apparatus (not shown) associated with the higher pressure compression part 4.
In the multistage compressor mentioned above, air is compressed to an intermediate pressure in the lower pressure compression part 3, and the air compressed to the intermediate pressure is transferred through the intermediate conduit 5 to the higher pressure compression part 4 where the gas is compressed to a higher pressure. The resultant high pressure compressed gas is transferred to and temporarily stored in the tank 7 and is then supplied to compressed gas-actuated machines to actuate them.
In the multistage compressor mentioned above, when the compressor is started or re-started after a long pause, aqueous vapor contained in the air can condense into waterdrops in the intermediate conduit 5 due to the difference of temperature existing between the intermediate conduit 5 and the compressed gas introduced into the conduit 5. The resultant waterdrops can enter the crankcase of the compressor, where the water mixes with lubricant in the crankcase to cause the lubricant to be emulsified.
Supposing that air in a high temperature and high humidity condition of, for example, 30.degree. C. and 90% humidity, is sucked into the lower pressure compression portion 3 of the compressor shown in FIG. 1, and that the pressure of the gas in the intermediate conduit 5 is 2.5 kg f/cm.sup.2, the dew point will be 52.degree. C. In this case, when starting or re-starting after a long pause of between thirty minutes and over one hour, which can occur during an extreme intermittent operation of the compressor due to a small amount of compressed air being consumed, the temperature of the intermediate conduit 5 has been lowered below 52.degree. C. When compressed air touches the intermediate conduit 5 of such lowered temperature, drainage is created. The drainage can flow into the crankcase, in which the drainage can mix with lubricant therein to emulsify it.
In order to deal with this problem, the following steps of operation have conventionally been taken in the multistage compressor mentioned above. Prior to the starting or re-starting of the compressor after a long pause, the lower pressure side unloader apparatus is manually operated to bring the lower pressure side compression part 3 into the non-compressing condition. Only the higher pressure side compression part 4 is actuated to compress gas, for a while, and then, after the compressor body is warmed up to a certain extent, the lower pressure side unloader apparatus is stopped, so that the lower and higher pressure compression parts 3 and 4, respectively, are both actuated to compress, thereby preventing the lubricant in the crankcase from being emulsified.
As explained above, in the multistage compressor shown in FIG. 1, the lower pressure side unloader apparatus is actuated so that compression of air is only effected by the lower pressure side compression part 3, in order to prevent emulsification of the lubricant in the crankcase. As a result, the volume of air to be compressed is about one fourth of that in the case in which the lower and higher pressure side compression parts 3, 4 are both actuated to compress air, thereby lowering operation efficiency.
In order to solve the problem mentioned above, an arrangement as shown in FIG. 2 has been proposed. The arrangement comprises a cooler 8 disposed midway of the intermediate conduit 5 for cooling compressed gas flowing from the lower pressure side compression part 3 to the higher pressure side compression part 4. The cooler 8 includes a cooling body 9 which cools gas by causing heat to radiate from the gas, or by using a refrigerant and a drain separation chamber 10 disposed downstream of the cooling body 9.
An obstacle plate 11 is disposed in the drain separation chamber 10 opposite the cooling body 9. The drain separation chamber 10 is further provided with an outlet port 12 for discharging the drainage. At the drain outlet port 12 is disposed a release valve 15 including a valve body 13 and a spring 14 for normally biasing the body 13 to open the valve 15 and adapted to be compressed to close the valve 15 when pressure in the intermediate conduit 5 reaches a predetermined value which is substantially equal to the intermediate pressure of the multistage compressor.
In the multistage compressor provided with the arrangement mentioned above, air which has been compressed in the lower pressure side compression part 3 is cooled by means of the cooling body 9 to intentionally create drainage. The resultant drainage is in turn blown onto the obstacle plate to be separated from the air and directed to the bottom of the drain separation chamber 10 where the drainage is discharged from the chamber 10 through the release valve 15, thereby preventing condensed waterdrops from entering the crankcase and emulsifying the lubricant therein.
The multistage compressor provided with the above-mentioned arrangement for discharging drainage, however, suffers from the following problems.
In the multistage compressor mentioned above, the pressure in the intermediate conduit 5 reaches the aforementioned predetermined value, which is set near the intermediate pressure of the compressor, just immediately after the compressor is started so that the release valve 15 is closed before the temperature of the intermediate conduit rises over the dew point. Thus, the valve 15 is only opened during a very short period of time, allowing only a very small amount of water to discharge as drainage and waterdrops which are created during the period of time from the closing of the release valve 15 to the rising of the temperature of the intermediate conduit 5 over the dew point in the drain separation chamber 10. The accumulated water or drainage may possibly evaporate again during the following compressing operation of the compressor, thereby obstructing reliable prevention of emulsification of the lubricant in the crankcase.